Nation of the Sun by HR Moore

Their demon lives spun around each other. Their souls, like magnets, pulled back together in every lifetime.

Amari has a perfect life. She’s a successful food critic, and is marrying a high flying human rights lawyer. But the day before her wedding, a stranger, Caspar, tries to solicit her help. She sends him away, but can’t shake the feeling that she knows him.
When Amari’s new husband has to leave the country before their honeymoon, Amari tells Caspar she’ll help him. But Amari and Caspar are attacked by an assassin, forcing them into hiding at the London headquarters of the Pagan Nation. Here, Caspar tells her that she’s an ancient and powerful demon, someone who reincarnates, and that he is her soulmate.
As she’s drawn into Caspar’s world of standing stones and feuding nations, Amari can’t deny the deep connection that pulses between them. But she can’t remember her past, and the Pagans have secrets they refuse to reveal: Why did Amari avoid Caspar for a hundred years? And what happened between Amari and the leader of a rival nation in the past? To determine if she can trust Caspar, if she should help him, she must wake her demon soul, and bring back her memories. For one thing is certain: when that happens, the tables will turn.

Trigger warnings for violence and kidnap. Contains sexual content.